Immature Stages of Braconidae
Immature stages of Braconidae were discussed in detail by Clausen (1940), as follows:
The Egg.--The general form of the eggs of the Braconidae is simple, ranging in outline from broadly oval to almost cylindrical but frequently somewhat pear‑shaped, or elongate and tapering at both ends and usually without a stalk or pedicel. The egg of Microbracon lendicivorus (Fig. 13B) differs from those of other Vipioninae in having a slender tapering stalk, slightly longer than the egg body, at what is presumably the anterior end. In Dendrosoter protuberans Nees (Fig. 13G), the stalk is very broad and bent back upon the egg body in a characteristic way, whereas, the stalk of Coeloides subconcolor Russo (Fig. 13F) is long and has a distinctly segmented appearance (Russo, 1938). Most of the species of Apanteles have a short peduncle at the posterior end, and Opius crawfordi Vier. has this peduncle equal to or longer than the egg body. Other species of the latter genus lack the peduncle entirely or have it in a very reduced form (Fig. 13D, E). A thin, transparent membrane possibly the exochorion envelops the egg of O. tryoni, which is broken during the period of incubation owing to increase in size of the egg. This recalls a similar egg envelope found by Dowden in Brachymeria compsilurae Cw.
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The Euphorinae have the stalk at the posterior end, and in some species of Perilitus it is nearly as long as the main body. The same is true in a number of species of Meteorinae. The egg of Alysia manducator (Fig. 13C) bears a pronounced buttonlike tubercle at its larger, presumably anterior end. Although the evidence is not complete and there are several apparent exceptions, it appears that tho stalk of the braconid egg, when present, is usually situated at the posterior end. In no instance does it serve any definite purpose after deposition.
The eggs of Ascogaster, Phanerotoma, and Chelonus, which are dcposited in those of the host, are of minute size, measuring 0.2 mm. or less in length. In all species of the family the chorion is thin and transparent, and usually has no surface sculpturing, though in Meteorus versicolor and Microdus dimidiatus it bears minute hexagonal markings.
First‑instar Larvae. --The first‑instar larvae of the Braconidae represent a considerable variety of forms, comprising the hymenopteriform, mandibulate, caudate, vesiculate and polypodeiform. It is often debatable; particularly in reference to this family, as to whether a particular larva should be classified as mandibulate, caudate, vesiculate, or polypodeiform, for it may possess two, or in some cases three, of the characters upon which the grouping is based. The hymenopteriform larva has a medium‑sized head, 13 body segments, which usually bear transverse bands or rows of setae, and spiracles on the first thoracic and the first eight abdominal segments. This type of larva is representative of the ectophagous forms, comprising the Vipioninae and representatives of the Braconinae and Horminae, and will doubtless be found in other groups. Bracon sp., probably B. hylobii Ratz., is distinguished by the lack of spiracles (Munro, 1917).
The mandibulate larvae are found generally in the Opiinae and, in combination with the caudate character, in the Euphorinae, Triaspinae, Alysiinne, and Pambolinae. Vesicle‑bearing larvae of this type occur in the Macrocentrinae. The larva of Opius tryoni (Pemberton and Willard, 1918) is typical of the Opiinae and has a large, heavily sclerotized head, large falcate mandibles, and short, blunt antennae. A pair of fleshy finger‑like processes is found ventrally at the anterior margin of the first thoracic segment. A well‑defined tracheal system, with anterior and posterior commissures, is present and filled with air, but there are no spiracles. The larvae of O. humilis, O. fletcheri (Fig. 15), and O. fullawayi are very similar to that of O. tryoni. Keilin and Picado (l913) have made an extended study of the first‑instar larva of O. crawfordi (Fig. 16A), which has an almost spherical head, a pair of very large mamma‑like processes, each surmounted by three sensory papillae, on the first thoracic segment and a smaller conical‑shaped pair on the third segment. These processes are on the concave side of the body, as is also the anal opening. The authors assert that the concave side of the body, to which the mouth opening is directed, is in reality the dorsum and support their conclusion by demonstrating the presence of the nerve cord along the convex side and of the heart on the concave side. Recognition of the markedly concave side of the first‑instar larva as dorsal rather than ventral is also reported by Baume‑Pluvinel (1914) in Adelura gahani B.‑P. (Fig. 16B), which develops in the larvae of various Phytomyzinae. Further investigation of this interesting point in other species would be desirable.
The larva of Ascogaster quadridentata has an exceedingly large head and 13 body segments of diminishing width, with no tail or other fleshy processes, whereas Chelonus annulipes, which is of modified mandibulate form, has a short tail following the 8 or less distinct body segments. The larva of Macrocentrus ancylivoruss is elongate in form and has a pair of short fleshy processes and a short caudal horn on the last segment. The anal vesicle is relatively small. In the Euphorinae the large, heavily sclerotized head, bearing the falcate mandibles at the front, is followed by 12 or 13 body segments of decreasing width and a rather short tail, which bears setae on the distal half or two‑thirds.
The true vesticulate larvae are found mainly in the subfamily Microgasterinae, of which the principal studies have been made in the genera Apanteles and Microgaster. At the time of hatching, many of these have the general appearance of mandibulate larvae, and they may bear a fleshy tail approaching half the length of the body proper. Usually only 10 or 11 ring-like body segments are distinguishable, the last segment apparently representing several that have fused. Each of the segments usually bear a transverse row of setae dorsally. In A. tasmanica Cam. (Dumbleton 1935) and Miscogaster tibialis (Fig. 17a) (Vance, 1932a), the rows of setae are lacking on the first two segments, while in other species they are missing on only the first segment. The vesicle appears shortly after feeding begins, and its width is then equal to or greater than that of the preceding segments (Fig. 17C). The body at this time is somewhat cylindrical, and the tail, which previously was prominent, now appears as only a small ventrally directed "horn" beneath the vesicle. In A. militaris (Tower 1915), A. hyphantriae Riley, and A. thompsoni, there is no indication of a tail structure at any time, and the bulb-like vesicle is well developed even before hatching.
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The wall of the proctodeum of the first‑instar larva of Orgilus obscurator Nees is relatively thin (Fig. 18), but it increases greatly in thickness in the second instar (Thorpe 1932).
The simple caudate type of larva, without other adaptive modifications, is found principally in the Meteorinae and Aphidiinae, whereas the tail in some form is present in practically all groups which develop internally. In Meteorus, there are 12 or 13 segments exclusive of the tail, and the latter may exceed the body in length, though in some species it is only one‑eighth as long. Each segment usually bears a transverse row of setae on the dorsum, and the tail may also bear setae. The first‑instar larvae of many of the Aphidiinae are recognizable principally by the possession of a row or comb of heavy setae at the posterior margins of each body segment and by the two ventrally directed lobes of the last segment. The tail in this subfamily is usually somewhat tubular in form with the distal end rounded. There are 13 distinct body segments. The fringe of spines at the posterior margin of each segment dorsally and extending to the lateroventral margins is, so far as known, found only in Praon, while in P. simulans, studied by Timberlake (1910), they occur only on the third thoracic segment and on all abdominal segments except the last. Janiszewska (1933) describes the larva of an undetermined Aphidiine believed to be Aphidius, in which this row or comb of spines is present on each body segment. In Ephedrus incompletus Prov. (Wheeler 1923) and other species of the genus, the larva (Fig. 19) bears on each segment a median transverse ridge which is more pronounced on the dorsum and sides and is strongly serrate, with the teeth directed caudad. The tail also is heavily and completely spined, with the spines arranged in rings about it. These adaptations are possibly locomotory in function. The ventrally directed bilobed process of the caudal segment is found in Praon, in Ephedrus, and in some species of Aphidius, and consists of two conical or finger‑like processes, about the length of one segment, situated ventrally at the base of the tail. The majority of the species of the genus Aphidius have simple caudate larvae, which lack entirely the integumentary spines and the paired caudal process, and the tail is only lightly spined on its distal portion. Larvae of the Aphidiinae have the anal opening ventrally at the base of the tail rather than dorsally; in the species having the lobed processes, it is situated between the bases of the lobes and the tail.
The polypodeiform larva is found in isolated species in a number of subfamilies. That of Dacnusa navicularis var. cynaraphila Ric. (Ricchello 1928) (Fig. 20A) is, except for its paired ventral processes, typically caudate, with a transverse row of setae dorsally on each abdominal segment. The paired ventral processes occur upon each of the 12 body segments and are surmounted by a group or row of setae. The larva of D. areolaris (Fig. 20D), on the other hand, lacks the tail and the paired ventral processes (Haviland, 1922a). Bassus dimidiator (Fig. 21) (Silvestri, 1923a), B. pumilus Ratz. (Thorpe, 1933), B. stigmaterus Cress, and Macrocentrus gifuensis are distinguished by having two pairs of fleshy processes on each segment. In M. gifuensis, these are present on the first 12 segments and are of uniform size, whereas in the first two species named they are lacking on the first segment and are of slightly greater size on the abdomen.
The respiratory system of first‑instar larvae of certain of the endoparasitic species, such as the Opiinae, consists of the two lateral trunks with branches at the various segments and an anterior dorsal and a posterior ventral commissure. In a considerable number of species, however, there is a complete lack of the tracheal system in this instar.
n the great majority of species, the mandibles are simple, though several exceptions occur. Those of Microbracon brevicornis are dentate on the lower border, whereas in Bracon tachardiae they are dentate, the teeth being long and spine‑like, and in Heterospsilus cephi (Hill and Smith, 1931) the main tooth is followed by five or six elongate teeth in comb‑like arrangement along the inner edge. It will be noted that these departures from the normal are in species which feed externally.
Intermediate‑ and Final‑instar Larvae. --The intermediate larval instars of the ectoparasitic species do not differ in any essential character from the first instar. Among the internal parasites, the mandibulate‑type larva loses the large, heavily sclerotized head, with its long falcate mandibles, at the first molt, and in the caudate forms the tail is reduced in size with each succeeding molt and is entirely lacking in the last instar. In some species having five instars, it disappears after the second molt, and in Cosmophorus henscheli it is entirely absent in the second and following instars.
Many first‑instar Apanteles larvae have a tail, or "caudal horn," situated beneath the vesicle, which in some species disappears entirely at the first molt and in others persists in reduced size in the second‑instar larva. In contrast to this, the anal vesicle of the larvae that possess it increases in size with each molt but is absent in the final instar. In A. thompsoni, it is said to be present for only a short time after the second molt. The paired ventral processes on the last segment of many aphidiine larvae do not persist beyond the first instar. Many species reveal an increasing number of small teeth on the inner margin of the mandibles in the intermediate instars. In Bracon tachardiae Cam., the four teeth of the first instar are succeeded by five in the second and third, whereas the mandibles of Microbracon mellitor Say are simple in all instars.
The paired ventral processes that occur on the bodies of first‑instar polypodeiform larvae, such as those of some species of Bassus and of Macrocentrus gifuensis, persist in much reduced form in the second instar. According to Parker, the larva of the latter species lacks mandibles in this instar.
The mature larvae of the Braconidae are of normal form and have few characters that distinguish them, aside from the tracheal system. In many species, the mandibles have minute teeth, often slender and spine‑like, on the inner margin, approaching 30 in number in some species, giving a comblike appearance. Voukassovitch, in describing the mature larva of M. abdominalis mentions a bilobed chitinous "anal capsule," of which the ventral lobe is more heavily sclerotized and bears a small ventrally directed process. The anal opening is between the two lobes of the capsule. Beeson and Chatterjee refer to a prominent "process" ventrally on the fifth and sixth abdominal segments of the larva of Perilitus mylloceri Wlkn. but do not otherwise describe it.
The mature larvae of many of the ectophagous species bear a dense coating of fine hairs; in some instances, this is uniform over the body, and in others it occurs in a transverse band on each segment and may be absent ventrally.
It has already been pointed out that the respiratory system of the ectoparasitic first‑instar braconid larva has normally nine pairs of spiracles, situated on the first thoracic and the first eight abdominal segments In these species, largely included in the Vipioninae, this number and arrangement persist through all the following instar. The early‑instar larvae of the species that develop internally lack the open tracheal system; and, in species that are known to have five instars, the spiracles first appear on the fourth. The species in which only three or four larval instars have been distinguished reveal the spiracles only on the last instar.
De Leon (1934) has summarized the information available regarding the respiratory system of mature braconid larvae and has attempted to group the subfamilies on the basis of spiracle number and position and on the presence or absence of certain commissures. The information available is sufficient for only a very few subfamilies to permit of generalizations in this respect. It appears, however, that the Vipioninae quite consistently have the number and arrangement given above, and limited information indicates that this is true of the Braconinae also. The most common spiracular arrangement, however, has the same number, but the thoracic pair is situated on the second segment rather than the first. This order appears to predominate in the Macrocentrinae, Meteorinae, Euphorinae, Opiinae, and Alysiinae. Macrocentrus ancylivorus is said to have the spiracles on the second and third thoracic and the second to eighth abdominal segments, whereas M. abdominalis has 10 pairs, the additional one being upon the ninth abdominal segment. In the Microgasterinae, the occurrence of eight pairs is quite general, and in most species the single thoracic pair is on the second segment, whereas in a smaller number it is upon the third. Microgaster connexus, however, has only six abdominal pairs rather than seven, and Apanteles lictorius Rein. is said to have nine pairs, though their position is not given.
On the basis of information regarding a limited number of species, it seems that the greatest variation in spiracle arrangement occurs in the Aphidiinae. Aphidius granarius L. has spiracles on the first thoracic and eight abdominal segments, and Ephedrus plagiator Nees (Skriptshinskij, 1930) on the second and third thoracic and seven abdominal segments. Wheeler (1923) states that tracheal system and spiracles are absent in the aphidiine species studied, representing three genera but this is so unusual as to require verification.
The tracheal system of the mature braconid larvae is distinguished from that of the Ichneumonidae chiefly by the absence of the secondary lateral commissures in the thorax, which connect with the main trunks by three branches. The anterior dorsal commissure is present in all species, but the absence of the posterior ventral commissure has been noted in species of Chelonus, Apanteles, Microplitis, and Meteorus. Ventral abdominal commissures occur in the first eight segments in several species of Vipioninae and in Doryctes gallicus Rh.
Among the species that develop within the host, many have an internal tracheal system but no spiracles in the early instars, and the spiracles appear only in the last larval instar. In Bracon sp. (probably B. hylobii) studied by Munro (1917), which is the single species of external habit that lacks spiracles in the early instars, they appear first in the fourth (penultimate) instar.
According to Glover (1934), who has studied the immature forms of B. tachardiae, the head widths of the five larval instars conform to Dyar's principle, though the extremes overlap, whereas mandible length shows no overlapping.
De Stephani Perez (1902) has described the so‑called chrysalis of Giardinaia urinator Perez, found upon the stems of Potamogeton, which he considers to be the last larval exuviae and within which pupation is said to take place. The last body segment is bifurcate, and the terminal "hooks" are embedded in the stem of the plant. They may bear spiracles, and the supply of air would thus be derived from the plant. The figure of this chrysalis shows 14 segments. The parasite pupa illustrated within it is slightly more than half its length and one‑sixth to one‑eighth its volume. At emergence, the wasp breaks through the dorsum of the chrysalis and climbs to the surface of the water. It is extremely improbable that this chrysalis is the last larval skin of Giardinaia; it is much more likely to be that of its host. This parasite has been recorded from Hydrellia sp. in Europe and the "chrysalis" described by Perez may be the empty larval skin of a species of Ephydridae. The aquatic larvae of some representatives of this family are known to possess caudal spiracles upon a bifurcate process and to derive their air supply from plants (Clausen 1940).